As a result of human activity there have been accidental, deliberate or unknowing releases of petroleum products, chemicals, metals and hazardous, toxic and radioactive substances (hereinafter referred to as contaminants) into the environment. In many of these cases, these releases have been to the terrestrial environment. Upon contact with soil, rock and other solids of the terrestrial environment, these contaminants mix and adsorb making recovery of the contaminants extremely difficult. Left unrecovered these contaminants may be transported to the biosphere through surface or ground water, or through direct contact, resulting in a hazard to living organisms, including human beings. There are increasing worldwide requirements to clean or "remediate" contaminated terrestrial solids by recovering these undesirable contaminants and safely disposing of them.
Current technologies for remediating contaminated soil fall into two broad categories: in-situ, those technologies which extract the contaminant while the soil remains in-place; and ex-situ, those technologies which excavate the contaminated soil and treat that soil by surface process technologies. While in-situ technologies have proven useful for certain types of contaminants such as volatile organics, in-situ technology has not proven to be generally applicable to a broad range of undesirable contaminants.
Among the ex-situ technologies, there are two generic approaches: thermal and water-washing. Thermal processes rely on distillation, pyrolysis and/or combustion of the contaminants. Thermal processes are applicable only to organic contaminants and are ineffective for remediation of soils with metal or radioactive contaminants. Thermal processes are inherently energy-intensive, resulting in high costs. Thermal processes also have air quality implications, requiring air quality permitting.
Washing of soil with water has been previously proposed, in for example U.S. Pat. Nos. 5,056,542, 4,951,417, 4,783,263, and "Soil Washing Results of EPA Tests for Effectiveness" The Hazardous Waste Consultant (May/June) 1989 pp 1-11 to 1-16. In general, proposed soil-washing processes rely on technologies largely developed in the extractive metallurgy industry. Common to these approaches are a form of pretreatment in which surfactant, caustic or other ingredients are added to water and soil and the components are mixed in a stirred tank. The resulting pre-treated mixture is then sent to an air flotation device in which the organic material is caused to float while the heavier-than-water ingredients such as minerals and soil, sink to the bottom and are removed.
Water washing technologies, using traditional extractive metallurgy configurations, are generally limited to petroleum hydrocarbons and other insoluble organics whose density is approximately equivalent to, or less than, that of water. These technologies are not by themselves applicable to water-soluble contaminants, which require additional soil washing and water processing steps for removal.
Many of the difficulties with current soil-washing technologies relate to the requirements for a large pre-treatment vessel and multiple flotation stages. The stirred-tank process design inherently requires multiple stages, often with long residence times, to achieve a high-level of separation of the contaminants from the soil. Further, the current soil washing processes separate primarily water-insoluble organic contaminants. Therefore, any soluble mineral or soluble organic contaminants remain with the water and are discharged, along with the soil in the underflow of the flotation cells. A further process is accordingly required to further decontaminate the underflow from the flotation stages.
A further problem of currently-practiced technology is the need for large volumes of water. Even though water is recycled, the overall volume of water in the recycle loop may be several times the inventory of the soil in the process. These high water-to-soil ratios, along with the mixing of fresh water in the downstream settlers and thickeners, tend to dilute the recycle streams and make maintenance of process water quality difficult. In addition, the high ratio of water to soil and the inherent inflexibility of multiple, stirred-tank reactor stages render it difficult to control the process to achieve a high degree of remediation. In addition, the contaminate in aqueous outflow from the cells is often highly diluted. For these reasons, the outflow often requires a further cleaning and often requires large water processing facilities.